Method and measuring apparatus for measuring freeness

ABSTRACT

The invention relates to a method and measuring apparatus for measuring freeness. Freeness is measured with the help of an automatic data processing apparatus ( 30 ) so that the flow rate of a liquid draining from a measuring chamber ( 10 ) is measured using the measuring apparatus ( 12 ) until the flow rate is equal to or lower than a predefined flow rate.

FIELD OF THE INVENTION

The invention relates to a method for measuring freeness, in whichmethod a measuring chamber comprising a wire or the like arranged on thebottom surface of the measuring chamber is filled with a suspension tobe measured and the suspension is allowed to flow through the wire orthe like at a time instant T0.

The invention further relates to a measuring apparatus to measurefreeness, which measuring apparatus comprises a measuring chambercomprising a top lid and a bottom surface shutter that tightly seal themeasuring chamber, an air valve and a wire or the like; in the beginningof the measuring process, the measuring chamber contains the suspensionto be measured; the shutter of the measuring chamber bottom surface ismade to be opened; the measuring apparatus is adapted to allow thesuspension to flow through the wire or the like at a time instant T0.

BACKGROUND OF THE INVENTION

To make good quality paper, the properties of paper stock must beprecisely measured and controlled. In measuring the freeness of paperstock, the speed with which the paper stock can be separated from wateris empirically determined. Freeness depends on several factors, such asfibres, stock processing (for instance mechanical/chemical), thequantity of fines, temperature, consistency, and the measuringapparatus.

One of the most common methods for measuring freeness is CSF (CanadianStandard Freeness). This measuring method is a standard and has beendisclosed in detail in publication T 227 om-85, Freeness of pulp, TAPPI,1985, which is incorporated herein by reference. In the CSF measurement,the freeness of paper stock is measured from a sample with 0.3%consistency and 20° C. temperature. If the consistency or temperature ofthe sample differs from the specified values, the freeness result isadjusted according to predefined table values so that the measurementcorresponds to the specified consistency and temperature values. In thebeginning of the CSF measurement, exactly one litre of the sample ismeasured into a measuring tank comprising the walls of the tank, a toplid that closes against the top part of the walls, a wire at the bottomof the tank, a bottom lid that closes against the bottom part of thewalls, and an air valve. The bottom lid is opened and the sample isallowed to settle in the tank so that some of the stock descends on thewire at the bottom of the tank. After approximately 5 s from opening thebottom lid, the air valve is opened so that water starts separating fromthe stock sample through the wire and the stock piled on the wire. Thewater flows into a funnel comprising a constant flow spout at the bottomof the funnel and a lateral tube in the bottom section of the funnel. Aconstant volume (24.2 ml) remains in the funnel between the constantflow spout (constant flow 8.83 ml/s) and the lateral tube. When waterflows from the measuring tank into the funnel, part of the water flowsout through the constant flow spout, a constant volume (24.2 ml) ofwater collects between the constant flow spout and the lateral tube andfinally some of the water flows out through the lateral tube. Inmeasuring freeness, this volume of water that has flown out through thelateral tube is measured in a measuring glass and this volume of watercorresponds to freeness. The measuring is usually performed manually.The measuring method is arduous and sensitive to changes in temperatureand consistency. The measuring method is also inaccurate with low CSFvalues.

Another known method for defining freeness is the Schopper-Rieglermethod disclosed in publication SCAN-C 19:65, Scandinavian pulp, paperand board, Testing committee, approved 1964, which is incorporatedherein by reference. According to this standard method, a known quantityof paper stock is first poured on a spreader cone which is opened aftera predefined period of time (5 s), the stock is filtered through a wireand a mat of fibre piling on the wire into a funnel with an orifice atthe bottom and the side. Water flows out through the bottom orifice at aconstant flow rate [1000 ml/(149 s±1)≈6.71 ml/s]. A constant volume (7.5ml-8.0 ml) remains between the bottom orifice and the side orifice. Thevolume of water flowing through the side orifice corresponds to freenessmeasured in SR units so that 0 ml corresponds to 100 SR units, 1000 mlcorresponds to 0 SR units and thus one SR unit corresponds to 10 ml. TheSR and CSF scales are reversed in relation to each other, i.e. thehighest SR value corresponds to the lowest CSF value. This measuring isalso usually performed manually. The measuring method is arduous andsensitive to changes in temperature and consistency. The SR measuringmethod is also inaccurate with extreme values.

Patent publication U.S. Pat. No. 2,602,325, which is incorporated hereinby reference, discloses a method for measuring freeness similar to CSFmeasuring, in which freeness is determined by measuring the time thatelapses when a predefined volume of liquid separates from a suspensionin a measuring chamber. Alternatively, it is possible to measure thevolume of liquid that separates from a suspension in a measuring chamberduring a predefined time. This solution provides the disadvantage ofbeing slow and susceptible to human error.

Freeness can also be measured with an automated measuring apparatuswhich resembles CSF measuring in principle, but uses vacuum. A samplingapparatus of the measuring apparatus takes a sample of paper stock, andwater is added to the sample so that its consistency becomesapproximately 3%. The temperature of the sample is measured and thesample is allowed to descend and settle on the wire for 5 seconds. Afterthis, the water is drained from the tank through the wire using vacuum.After a certain period of time, the pressure difference caused by thecake piled on the wire is measured. After the pressure difference hasbeen measured, the consistency of the paper stock is measured using themass of the cake. Freeness can be calculated from the pressuredifference. The disadvantage of this measuring method is that patternmaking (calibration) is arduous and difficult, because the apparatusmust be calibrated separately for each stock type.

Patent publication FI 80 342, which is incorporated herein by reference,also discloses an automated method and apparatus for defining the drystuff, freeness and wire retention of pulp. The measuring of freeness isbased on measuring the liquid level of a suspension in a measuringchamber as a function of time. The drainage rate is formed as a functionof the mass of the dry stuff cake. The measuring requires a preciseweighing machine so as to avoid errors in the freeness result. Aweighing machine also increases the manufacturing costs of theapparatus.

Patent publication FI 51 133, which is incorporated herein by reference,discloses another automated method and apparatus for measuring freenessby directing pressurized water through a layer of stock and determiningthe freeness resistance by the volume of water that flows through duringa certain period of time. The disadvantage of this measuring method,too, is that calibration is arduous and difficult, because the apparatusmust be calibrated separately for each stock type. Pressurized measuringdiffers from standard measuring and the results are thus not comparable.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to solve the above-mentioned problemsby implementing a method and an apparatus implementing the method. Thisobject is achieved by a method described in the introduction andcharacterized in that in the method, when the flow starts at a timeinstant T0, the decrease in the suspension in the measuring chamber ismeasured as a function of time; a time instant T1 is searched for, atwhich the decrease in the suspension substantially corresponds to apreviously known flow rate v_(c); and freeness F is determined as afunction of the volume of suspension drained from the measuring chamberby the time instant T1.

The measuring apparatus of the invention is characterized in that itcomprises a measuring sensor for measuring the drainage of a liquid froma measuring chamber through a wire or the like as a function of time;the measuring apparatus comprises an automatic data processing apparatusto which the sensor is made to feed its measuring data; the automaticdata processing apparatus is adapted to search for a time instant T1 atwhich the drainage of the liquid from the measuring chambersubstantially corresponds to a previously known flow rate v_(c); and theautomatic data processing apparatus is adapted to determine freeness Fas a function of the volume of suspension drained from the measuringchamber by the time instant T1.

The method and system of the invention provide several advantages.Measuring freeness becomes faster, more accurate and simple. Measuringdoes not require calibration and the measuring apparatus need not be setto the type of stock used in measuring. Further, repeatability improves,as the human factor involved in the measuring can be minimized. Themeasuring apparatus of the invention is also inexpensive and quick totake into use.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail inconnection with preferred embodiments and with reference to the attacheddrawings, in which

FIG. 1 shows a method of the invention,

FIG. 2 shows a measuring apparatus of the invention, and

FIG. 3 shows a freeness measuring method of the invention in relation toa standard measuring method.

DETAILED DESCRIPTION OF THE INVENTION

The solution of the invention is especially suitable for paper industry,but is not restricted to it.

Let us first briefly examine the method of the invention. In thesolution of the invention the volume of a liquid in a measuring chamberis monitored as a function of time from a time instant T0. When thederivative of the volume of the liquid in relation to time, whichcorresponds to the flow rate of the liquid draining from the measuringchamber through a wire, reaches a predefined value v_(c) at a timeinstant T1, a total volume of liquid V_(total) drained from themeasuring chamber up till then, i.e. between T1 and T0, is calculated. Avolume V_(cf) that would correspondingly have drained during the sametime T1 to T0 through a constant flow spout, i.e. V_(cf)=v_(c)*(T1−T0),is subtracted from the total volume V_(total). In the CSF method,constant flow v_(c) is 8.83 ml/s and in the SR method, v_(c) is 6.71ml/s, i.e. more specifically 1000 ml/T_(vc), where T_(vc) is 149 s±1 s.Additionally, a constant volume, which corresponds to a threshold valuestate V_(th) between a constant flow spout and a lateral tube of a lowerfunnel, is subtracted from the total volume. In the CSF method, thethreshold value state is V_(th)=24.2 ml and in the SR method, it isV_(th)=7.5 to 8.0 ml. Thus according to the method of the invention,freeness F is a function of the differences V=V_(total)−V_(cf)−V_(th),i.e. F=f(V), which corresponds to CSF and SR standard measuring. The CSFresult directly and substantially corresponds to the difference resultV, i.e. F=V. In calculating freeness F, it is, however, also possible touse some other suitable function based on practical experience. In theSR method, the SR scale increases in a different direction than in theCSF method, which means that freeness F in the SR method is nearlydirectly F=(1000 ml−V)/10 or some other suitable function based onpractical experience. The dependency between the SR and CSF methods isnon-linear due to, for instance different wires and air cocks when asuspension is allowed to flow through a wire. In the method of theinvention, it is naturally possible to freely select any required valuesfor the constant flow v_(c) and the threshold value state V_(th).

Let us now examine the method of the invention using FIG. 1. Themeasuring apparatus of the invention implementing the method of theinvention comprises a measuring chamber 10 comprising a top lid 34 and abottom lid 16 that tightly seal the measuring chamber 10, and an airvalve 14. The measuring chamber is attached to a supporting structure40. When measuring is started, the measuring chamber 10 is filled with asuspension to be measured. The filling may be performed manually byopening the top lid 34 with a lever 32 and then pouring the suspensioninto the measuring chamber 10, or automatically through tube 22. Manualfilling is not recommended in industrial processes and thus a manualopening mechanism of the top lid is not essential. When the filling isperformed through tube 22, an automatic data processing apparatus 30,which is for instance a computer with a microprocessor, opens a valve 26and the suspension starts to flow into the measuring chamber 10. Whenthe measuring chamber 10 is full, a bottom lid 16 is opened with anopening mechanism 20. After the bottom lid 16 has been opened, the airvalve 14 is opened after a predefined delay, usually 5 s, at a timeinstant T0. Opening the bottom lid 16, measuring the delay andcontrolling the opening of the air valve 14 are precisely executed bythe automatic data processing apparatus 30. The measuring apparatuscomprises measuring means 12 for measuring as a function of time thedrainage of the liquid from the measuring chamber after the air valvehas been opened. The liquid flows through a wire or perforated plate 18leaving the solid matter in the suspension on the wire or perforatedplate 18. The outflow of the liquid is measured with a sensor 12comprising, for instance an optical or ultrasonic transmitter-receiverpair. The sensor 12 is connected to the automatic data processingapparatus 30. The measuring is performed, for instance so that theoptical or acoustic transmitter sends a measuring signal towards thesurface of the suspension, from which the signal is reflected to theoptical or acoustic receiver. When the location of the transmitter andreceiver and the travel time of the signal from the transmitter to thereceiver is known, it is possible to specify the level of the surface.The sensor 12 feeds the measuring data of the signal travel time intothe automatic data processing apparatus 30 which specifies the flowrate. The automatic data processing apparatus 30 determines freeness Ffrom the collected measuring data according to the method of theinvention.

Let us now examine the method of the invention using FIG. 2 which showstwo curves describing the drainage of the liquid from the measuringchamber 10. In the case of the upper curve, the liquid flows more slowlyout of the measuring chamber 10 than in the case of the lower curve(marked with a dash line). The upper curve shows how to read from thecurve the total volume V_(total) of drained liquid, the volume flowingout at constant flow V_(cf), freeness F according to the CSF method andthe time instant T1, at which the liquid is flowing out from themeasuring chamber at a previously known constant flow rate. The lowercurve only marks the time instant T1. The slope of the straight line,i.e. the derivative of the curve, shown as the tangent of the curve atthe location of the time instant T1 corresponds to the constant flowv_(c). Typically, the measuring chamber 10 has a capacity of 1000 ml,but the capacity of the measuring chamber 10 is not essential for thesolution of the invention. The automatic data processing apparatus 30searches for, on the basis of the measuring data, a time instant T1, atwhich the outflow of the liquid from the measuring chamber 10corresponds substantially to a previously known flow rate v_(c) which isthe constant flow 8.83 ml/s (in the SR method, the constant flow isapproximately 6.71 ml/s) of the constant flow spout of the lower funnelpreferably used in the CSF standard measuring of freeness. In the CSFstandard measuring, when 8.83 ml/s (in the SR method, 6.71 ml/s) ofliquid or less flows through the wire, liquid no longer flows from thelateral tube of the lower funnel to the measuring vessel measuringfreeness, but the liquid flows out through the constant flow spout. Tomeasure freeness, the automatic data processing apparatus 30 determines,in the solution of the invention, the total volume V_(total), of liquiddrained from the measuring chamber 10 from the time when the air valvewas opened at the time instant T0 to the time instant T1. The automaticdata processing apparatus 30 also calculates the constant flow volumeV_(cf) from the time instant T0 to the time instant T1 so that theconstant flow V_(cf) is V_(cf)=V_(c)*(T1−T0). In the CSF standardmeasuring, the constant flow volume V_(cf) corresponds to the volume ofliquid flown through the constant flow spout. The automatic dataprocessing apparatus 30 calculates freeness F of the suspension bysubtracting the constant flow volume V_(cf) and the previously knownthreshold volume V_(th) from the total volume V_(total) of liquiddrained from the measuring chamber 10. For instance, freeness F of theCSF method corresponds directly and substantially to the differenceresult V=F=V_(total)−V_(cf)−V_(th). The method of the invention issuitable for measuring freeness according to both the CSF standard andthe SR standard. When the measuring has been performed, the measuringchamber can be cleaned, for instance with pressurized water through atube 24 when its shutter 28 is opened. The opening of the shutter 28 ispreferably controlled by the data processing apparatus 30 which, in thesolution of the invention, also measures the temperature of thesuspension with a thermometer 36. The data processing apparatus alsopreferably measures the consistency of the suspension. The consistencyand temperature of the suspension being measured in the measuringchamber is controlled by the data processing apparatus 30. Theconsistency is preferably 0.3% and the temperature 20° C. When thetemperature and the consistency differ from the specified values, thedata processing apparatus 30 corrects the freeness resultcorrespondingly according to the CSF standard table. The data processingapparatus 30 is connected or integrated to the data network or controlsystem of the process.

FIG. 3 shows the freeness results F measured with a measuring apparatusof the invention on the vertical axis and the freeness results CSFaccording to the CSF standard on the horizontal axis. Freeness has beenmeasured extensively between 50 and 650. The correlation of the resultsis almost exactly one, i.e. the solution of the invention producesfreeness results very closely corresponding to the CSF standardmeasurements. In fact, the small differences in the results are causedby errors in the CSF standard measuring. As the CSF standard measuringis performed manually, small errors occur easily, for instance inmeasuring time (5 s), opening the bottom lid and reading the measuringresults, whereas the measuring of the invention is fully automated andperformed in exactly the same way every time.

Although the invention is described herein with reference to examples inaccordance with the accompanying drawings, it is obvious that theinvention is not to be so limited, but the invention may be modified ina variety of ways within the scope of the inventive idea disclosed inthe appended claims.

What is claimed is:
 1. A method for measuring freeness, comprising:introducing a suspension to be measured into a measuring chamber havinga wire arranged adjacent a bottom of the measuring chamber; allowingliquid of the suspension to begin to flow out of the measuring chamberthrough the wire at a time instant T0; measuring a decrease of volume ofthe suspension in the measuring chamber as a function of time after theflow starts at the time instant T0; searching for a time instant T1 atwhich the decrease in the volume of suspension in the measuring chambersubstantially corresponds to a predetermined flow rate v_(c); andcalculating freeness F as a function of the volume of suspension drainedfrom the measuring chamber between the time instant T0 and the timeinstant T1.
 2. A method as claimed in claim 1, wherein to calculatefreeness F as a function of the volume of suspension drained by the timeinstant T1: a total volume V_(total) drained from the measuring chamberis determined from the time instant T0 to the time instant T1; aconstant flow volume V_(cf) is calculated from the time instant T0 tothe time instant T1 based on the predetermined flow rate v_(c); adifference result V is calculated by subtracting the constant flowvolume V_(cf) and a predetermined threshold volume V_(th) from the totalvolume V_(total) of liquid drained from the measuring chamber, andfreeness F of the suspension is calculated as a function of thedifference result V.
 3. A method as claimed in claim 1, wherein a flowrate of the suspension draining from the measuring chamber iscontinually measured until the flow rate substantially corresponds tothe predetermined flow rate v_(c) or is lower than v_(c).
 4. A method asclaimed in claim 1, wherein temperature of the suspension is measuredand freeness F is adjusted according to the temperature.
 5. A method asclaimed in claim 4, wherein adjustment of the freeness F for temperatureis performed according to a standard table.
 6. A method as claimed inclaim 1, wherein consistency of the suspension is measured and freenessis adjusted according to the consistency.
 7. A method as claimed inclaim 6, wherein adjustment of the freeness for consistency is performedaccording to a standard table.
 8. A method as claimed in claim 1,wherein volume of the suspension in the measuring chamber is measured byacoustically measuring a height of the upper surface of the suspensionin the measuring chamber.
 9. A method as claimed in claim 1, whereinvolume of the suspension in the measuring chamber is measured byoptically measuring a height of the upper surface of the suspension inthe measuring chamber.
 10. A method as claimed in claim 1, wherein timeand volume measurements and calculation of freeness are performedautomatically using microprocessor control.
 11. A method as claimed inclaim 2, wherein the predetermined flow rate is v_(c)=8.83 ml/s and thepredetermined threshold volume is V_(th)=24.2 ml and freeness F issubstantially F=V.
 12. A method as claimed in claim 2, wherein thepredetermined flow rate is v_(c)=1000 ml/(149 s±1 s) and thepredetermined threshold volume is V_(th)=7.5-8.0 ml and freeness F issubstantially F=(1000 ml−V)/10.
 13. A method as claimed in claim 1,wherein the suspension to be measured remains in the measuring chamberfor a predefined time before being allowed to begin to flow through thewire.
 14. A method as claimed in claim 13, wherein the predefined timeis 5 s.
 15. A measuring apparatus for measuring freeness, comprising: ameasuring chamber for containing a quantity of a suspension to bemeasured, the measuring chamber comprising a top lid and a bottom wallthat tightly seal the measuring chamber, an air valve, and a wirearranged adjacent a bottom end of the measuring chamber such that liquidof the suspension will drain from the measuring chamber through the wirewhen the bottom wall is opened and the air valve is opened; a measuringsensor for measuring volume of the liquid drained from the measuringchamber through the wire as a function of time; and an automatic dataprocessing apparatus coupled with the sensor and operable to search fora time instant T1 at which the volume of the liquid drained from themeasuring chamber substantially corresponds to a predetermined flow ratev_(c) stored in the automatic data processing apparatus, and furtheroperable to determine freeness F as a function of the volume ofsuspension drained from the measuring chamber between a time instant T0at which the suspension first begins to drain from the measuring chamberand the time instant T1.
 16. A measuring apparatus as claimed in claim15, wherein the automatic data processing apparatus is operable tocalculate a total volume V_(total) of liquid drained from the measuringchamber from the time instant T0 to the time instant T1, to calculate aconstant flow volume V_(cf) from the time instant T0 to the time instantT1 based on the predetermined flow rate v_(c), to calculate a differenceresult V by subtracting the constant flow volume V_(cf) andpredetermined threshold volume V_(th) from the total volume V_(total) ofliquid drained from the measuring chamber, and to calculate freeness Fof the suspension as a function of the difference result V.
 17. Ameasuring apparatus as claimed in claim 15, wherein the measuringapparatus is operable to continually measure a flow rate of the liquiddraining from the measuring chamber and to determine when the measuredflow rate substantially corresponds to the predetermined flow rate v_(c)or is lower than v_(c).
 18. A measuring apparatus as claimed in claim15, further comprising a thermometer to measure temperature of thesuspension, wherein the thermometer is operable to feed measuring datato the automatic data processing apparatus and the automatic dataprocessing apparatus is operable to adjust freeness according to thetemperature.
 19. A measuring apparatus as claimed in claim 18, whereinthe automatic data processing apparatus is operable to adjust freenesswith respect to temperature according to a standard table.
 20. Ameasuring apparatus as claimed in claim 15, wherein the measuringapparatus is adapted to measure consistency of the suspension and theautomatic data processing apparatus is operable to adjust freenessaccording to the consistency.
 21. A measuring apparatus as claimed inclaim 20, wherein the automatic data processing apparatus is operable toadjust freeness with respect to consistency according to a standardtable.
 22. A measuring apparatus as claimed in claim 15, wherein themeasuring sensor for measuring the volume of the liquid drained from themeasuring chamber is operable to acoustically measure a height of theupper surface of the suspension in the measuring chamber.
 23. Ameasuring apparatus as claimed in claim 15, wherein the measuring sensorfor measuring the volume of the liquid drained from the measuringchamber is operable to optically measure a height of the upper surfaceof the suspension in the measuring chamber.
 24. A measuring apparatus asclaimed in claim 16, wherein the predetermined flow rate is v_(c)=8.83ml/s and the predetermined threshold volume is V_(th)=24.2 ml, and theautomatic data processing apparatus is operable to calculate freeness Fso that freeness F substantially equals the difference result V.
 25. Ameasuring apparatus as claimed in claim 16, wherein the predeterminedflow rate is v_(c)=1000 ml/(149 s±1 s) and the predetermined thresholdvolume is V_(th)=7.5-8.0 ml, and the automatic data processing apparatusis operable to calculate freeness F so that freeness F is substantiallyF=(1000 ml−V)/10.